The present application is generally related to anti-resonant membranes, structures for holding such membranes, and hybrid reflective and absorbing housing defined by such anti-resonant membranes and supportive structures.
Noise can be harmful in a number of environments. Noise is widely recognized as harmful and a source of pollution as noise can be highly penetrating and disruptive. People can experience negative effects from noise, such as hearing loss, nausea, high blood pressure, increased stress, and the like. Sources of noise pollution can frequently be caused by machines, such as motor vehicles, aircraft, trains, generators, grinders, blenders, microwave ovens, sump pumps, and many other machines. Many of such machines can emit noise at relatively constant levels for long periods of time.
Various attempts have been made to provide noise reduction. Many noise shielding solutions are directly tied to the mass of a barrier. In general, noise transmission is governed by the mass density law, which states that the acoustic transmission T through a barrier is inversely proportional to the product of wall thickness, the mass density, and the sound frequency. Increasing the sound reduction characteristics of such a barrier can require increasing the thickness or mass density of the barrier, which adds weight and size to the barrier, especially in the low frequency range.
Other attempts have been made to improve noise reduction. For example, U.S. Published Patent Application 2013/0087407 describes a tunable, high-bandwidth, anti-resonant membrane with a weight disposed in a center portion of the membrane. U.S. Published Patent Application 2010/0212999 is related to conventional Helmholtz resonators with at least one membrane wall for the resonating housing. However, the resonance of the membrane wall(s) does not include noise reflecting capabilities. U.S. Pat. No. 7,510,052 discloses a sound cancellation honeycomb based on modified Helmholtz resonance effect. However, the honeycomb based solution in U.S. Pat. No. 7,510,052 does not include noise reflecting capabilities. U.S. Published Patent Application 2008/0099609 discloses a tunable acoustic absorption system for an aircraft cabin that is tuned by selecting different materials and changing dimensions to achieve soundproofing for each position and specific aircraft. Although it describes the integration details of conventional barriers and absorbers, the structures disclosed in U.S. Application 2008/0099609 are heavy and bulky. U.S. Pat. No. 7,263,028 discloses embedding a plurality of particles with various characteristic acoustic impedances in a sandwich with other lightweight panels to enhance the sound isolation. Although it could be lighter or thinner than traditional solid soundproofing panels, it is still bulky and its soundproofing operating frequency is high, which makes it less effective for low-frequency operation. U.S. Pat. No. 7,249,653 discloses acoustic attenuation materials that comprise an outer layer of a stiff material that sandwiches other elastic soft panels with an integrated mass located on the soft panels. By using the mechanical resonance, the panel passively absorbs the incident sound wave to attenuate noise. The panel has a 100 Hz bandwidth centered around 175 Hz and is not easily tailored to various environmental conditions. U.S. Pat. Nos. 4,149,612 and 4,325,461 disclose silators. A silator is an evacuated lentiform (double convex lens shape) with a convex cap of sheet metal. These silators comprise a compliant plate with an enclosed volume wherein the pressure is lower than atmospheric pressure to constitute a vibrating system for reducing noise. To control the operating frequency, the pressure enclosed in the volume coupled with the structural configuration determines the blocking noise frequency. The operating frequency dependence on the pressure in the enclosed volume makes the operating frequency dependent on environment changes such as temperature. U.S. Pat. No. 5,851,626 discloses a vehicle acoustic damping and decoupling system. This system includes a bubble pack which may be filled with various damping liquids and air to enable the acoustic damping. It is a passive damping system dependent on the environment. U.S. Pat. No. 7,395,898 discloses an anti-resonant cellular panel array based on flexible rubbery membranes stretched across a rigid frame. However, the structures disclosed in U.S. Pat. No. 7,395,898 do not consider the vibration effects of the supportive frame which significantly affect anti-resonance of each cell in terms of frequency and effectiveness. Furthermore, the whole panel structure becomes an efficient sound transmission path especially at the resonance frequencies. The overall effects due to both the cell and frame dynamics could significantly affect the panel's acoustic behavior and counteract its noise insulation performance. This is particularly important in large-scale, lightweight, and compact acoustic barrier design.